DNA polymerase I and exonuclease V can be combined in the presence of DNA followed by DNase to form a high molecular weight form of DNA polymerase which uses native DNA in an ATP-stimulated DNA-polymerizing system. The synthetic complex may be related to a natural complex found in E. coli. The natural complex exists bound to DNA which is in turn bound to a sedimentable structure from which it can be released by incubating at 30 degrees C. Natural complex activity is present in a rec B21 C22 mutant which lacks exonuclease V activity. The DNA-stimulated ATPase activity of the rec BC enzyme may be essential for complex activity.